1/8 is not a valid (IPv4) source. Nowhere is specified that you could omit
trailing or leading numbers. Why don't you use...?
iptables -A test -s 1.0.0.0/8 -j ACCEPT
And from the manual:
-s, --source [!] address[/mask]
Source specification. Address can be either a
hostname, a network name, or a plain IP address.
The mask can be either a network mask or a plain
number, specifying the number of 1's at the left
side of the network mask. Thus, a mask of 24 is
equivalent to 255.255.255.0. A "!" argument before
the address specification inverts the sense of the
address. The flag --src is a convenient alias for
this option.

Note

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